[Paper Review] Grain Boundary Softening from Stress Assisted Helium Cavity Coalescence in Ultrafine-Grained Tungsten

W. S. Cunningham, Y. Zhang+4 more|arXiv (Cornell University)|Mar 27, 2023

Fusion materials and technologiesMaterials Science3 citations

TL;DR

This study demonstrates that grain boundary softening in ultrafine-grained tungsten arises from stress-assisted helium cavity coalescence at elevated temperatures and low fluence, where preferential bubble formation on grain boundaries leads to localized plasticity. Atomistic simulations confirm that stress concentrations drive cooperative deformation mechanisms—such as atomic shuffling, dislocation emission, and unstable twinning—accelerating coalescence and reducing hardness.

ABSTRACT

The formation of helium cavities in coarse-grained materials produces hardening proportional to the number density and size of the cavities and due to the interaction of dislocations with intragranular helium defects. In nanostructured metals containing a high density of interfacial sinks, preferential cavity formation on the grain boundaries instead produces softening and often attributed to enhanced interfacial plasticity. Employing two grades of ultrafine-grained tungsten, we explore this effect using targeted implantation studies to map cavity evolution as a function of the irradiation conditions and quantify its impact on the mechanical response through nanoindentation. Softening is reported at implantation temperatures above the threshold for preferential grain boundary cavity formation but at a sufficiently low fluence prior to the growth of intragranular cavities. Collective changes in the mean cavity size, density, and morphology beneath a residual impression on an implanted surface indicate that cavity coalescence accompanied the reduction in hardness. Complementary atomistic simulations demonstrate that, in tungsten grain structures exhibiting softening, grain boundary bubble coalescence is driven by stress concentrations that further act to localize strain in the grain boundaries through cooperative deformation processes involving local atomic shuffling and sliding, dislocation emission, and even the nucleation of unstable twinning events.

Motivation & Objective

• To investigate the mechanical softening mechanism in ultrafine-grained tungsten under helium implantation.
• To determine the role of grain boundaries in helium bubble nucleation and evolution under irradiation.
• To link observed hardness reduction to microstructural changes such as cavity coalescence and morphology evolution.
• To elucidate the atomic-scale mechanisms driving localized plasticity at grain boundaries using simulations.
• To quantify the threshold conditions (temperature, fluence) for softening onset via nanoindentation and implantation studies.

Proposed method

• Conducted targeted helium ion implantation on two grades of ultrafine-grained tungsten to control fluence and temperature.
• Performed nanoindentation to measure hardness changes as a function of irradiation conditions.
• Used transmission electron microscopy (TEM) to analyze cavity size, density, and morphology beneath indented surfaces.
• Applied atomistic simulations to model stress-driven cavity coalescence and deformation mechanisms at grain boundaries.
• Tracked collective changes in mean cavity size, density, and shape to correlate with mechanical softening.
• Analyzed stress concentration effects on grain boundary plasticity, including dislocation emission and twinning nucleation.

Experimental results

Research questions

• RQ1 At what temperature and fluence does grain boundary softening become dominant in ultrafine-grained tungsten?
• RQ2 How does stress concentration at grain boundaries influence helium cavity coalescence and plasticity?
• RQ3 What atomic-scale deformation mechanisms (e.g., dislocation emission, twinning) are activated during cavity coalescence?
• RQ4 Why does grain boundary helium accumulation lead to softening instead of hardening, unlike in coarse-grained materials?
• RQ5 How do changes in cavity morphology and density beneath an indented surface correlate with measured hardness reduction?

Key findings

• Softening was observed in ultrafine-grained tungsten at implantation temperatures above the threshold for preferential grain boundary cavity formation, but at low fluence where intragranular cavities had not yet formed.
• A reduction in hardness was directly correlated with collective increases in mean cavity size, density, and morphological changes beneath the nanoindentation impression.
• Atomistic simulations revealed that stress concentrations at grain boundaries drive cavity coalescence and promote cooperative deformation involving local atomic shuffling, dislocation emission, and nucleation of unstable twinning events.
• Grain boundary softening is attributed to enhanced interfacial plasticity, contrasting with the hardening typically seen in coarse-grained materials due to intragranular defect interactions.
• The onset of softening coincided with the emergence of interfacial strain localization, indicating a transition from defect-hardening to defect-softening mechanisms.
• Cavity coalescence was found to be the dominant mechanism behind the observed mechanical softening, rather than isolated bubble growth or dislocation pinning.